Lubrication is exceedingly important in prosthetic joints. Lubrication enables sliding motion between the prosthetic joints, which are often used to improve synovial and other joints.
The most common total joint replacements occur at the hip and the knee as discussed in Kurtz, S., Mowat, F., Ong, K., Chan, N., Lau, E., Halpern, M., “Prevalence of Primary and Revision Total Hip and Knee Arthroplasty in the United States from 1990 Through 2002,” Journal of Bone and Joint Surgery, July 2005, Vol. 87A:7, pp. 1487-1496. A combination of limited prosthesis lifetime, an increasingly aging population, and the fact that more patients are receiving joint replacements at a younger age necessitate investigation of improvements to joint performance on all fronts. Current thrusts in joint research are focused in the fields of materials science and materials selection, mechanical component design, and improving lubricity of prosthesis components when bathed in synovial fluid. Orthopaedic alloys used for prostheses currently include Cobalt-Chrome (CoCr), Stainless Steel, Titanium, and Cross-Linked Ultra-High Molecular Weight Poly-Ethylene (UHMWPE), as discussed in Revel, P. A., Ed., “Joint Replacement Technology,” (2008), Woodhead Publishing, Ltd., Cambridge, UK.
When implanted into a patient, lubrication is provided by synovial fluid, an ultrafiltrate of blood plasma containing proteins, phospholipids, lubricin, and other molecules which affect normal lubrication regimes and give synovial fluid its shear-thinning, non-Newtonian properties. Synovial fluid is produced by the synovial membrane in the joint capsule. Denaturation of synovial fluid proteins, primarily albumin, has been attributed to increased friction and heat generated at the metal/polymer interface, as discussed in Mishina, H., Kojima, M., “Changes in Human Serum Albumin on Arthroplasty Frictional Surfaces,” Wear, 256:655-663, 2008. The increased friction and heat generation result in increased wear, and have the overall effect of decreasing the useful life of prosthetic joints. In bovine synovial fluid, Bovine Serum Albumin (BSA) is the most abundant protein. It is also, by convention and because it is readily available and relatively consistent in formulation, the most common lubricant used for tribological testing of orthopaedic materials. Denatured albumin preferentially adsorbs onto hydrophobic surfaces and forms a compact, passivating layer that increases sliding friction leading to increased shear stress and greater wear, as discussed in Heuberger, M. P., Widmer, M. R., Zobeley, E., Glockshuber, R., Spencer, N. D., “Protein-mediated boundary lubrication in arthroplasty,” Biomaterials, 26:1165-1173, 2005 and in Roba, M., Naka, M., Gautier, E., Spencer, N. D., Crockett, R., “The Adsorption and Lubrication Behavior of Synovial Fluid Proteins and Glycoproteins on the Bearing-Surface Materials of Hip Replacements,” Biomaterials, 30:2072-2078, 2009. Glycoproteins present in synovial fluid adsorb onto the hydrophobic polymer surfaces by way of their hydrophobic backbone, presenting their hydrophilic side chains to form a hydrated boundary layer on the surface of the polymer. As such, due to the wide range of proteins found in synovial fluid, and the variability in surface chemistry of the implants, there are significant challenges associated with improving lubrication on the molecular level.
Fluorescence microscopy and gel electrophoresis have been used to investigate the ability of glycoproteins to adsorb onto UHMWPE and alumina in the presence of other synovial fluid proteins. The wide range of proteins present in synovial fluid, including albumin, glycoproteins, proteoglycans, and glycosaminoglycans (GAGs), should be included in any tribo-rheological characterization of nano- or micro-engineered coatings. Using synovial fluid in experiments may facilitate the prediction of the behavior of the surfaces in relation to in-vivo lubrication. Models of normal articulating joint lubrication suggest that a lubricating gel is formed from thickly concentrated hyaluronic acid molecules, which acts as a boundary lubricant preventing cartilage-to-cartilage contact very briefly during gait cycles, as discussed in Tandon, P. N., Bong, N. H., Kushwaha, K., “A New Model for Synovial Joint Lubrication,” International Journal of Bio-Medical Computing, 35:2, 125-140, 1994. A similar boundary layer is formed in a joint prosthesis from both normal and denatured proteins. Polymers are not good conductors of thermal energy, and, if a polymer bearing insert is used, as in a total knee replacement, this can result in heating and micro-melting at any points of contact where increased friction is observed due to denatured protein adsorption. This results in further increases to the rate of protein denaturation, and could suggest one mechanism leading to increased wear of the hydrophobic surface of the polymer.
There is a need for new and improved prosthetic joint components with extended useful life.